Phew, you asking a good 1 here Vultan, it all depends on the Parrot.
Mine is a Senegal they not big talkers anyway, he is 7 comming up to 8 and it took me 4 years to get him to say hello. Best if started when very young. African Greys are Re-knowned for being the biggest talkers. you have to keep saying it constantly, My parrot took about a year to say night night, when i cover him up at night, but i did this every time i covered him up, he soon became associated with the cover and words Night night, he now says them reguarly, and if he is not covered up after 7pm, he will constantly say night night untill he gets covered. Its down to routine really. More you keep doing it the quicker they pick it up. But in saying that each Bird is different, some can pick it up quite quickly others may take time. Patience is the Keyword here, as with any animal. (i must add thos with my senegal, they love making noises, my friend who comes to visit as taught my parrot how to make err umm farting noises, he picked this up in about 6 months.)
nasty habit of having a bath in his drinking water.!!
Can i add tho please, lot of people, tend to bombard their birds with different words each time, this confuses the bird. Best way is to start with 1 easy word such as hello, then move onto another word.